There are currently 33 public hydrogen stations in the U.S., according to the Department of Energy's Alternative Fuels Data Center.

The vast majority of those are in California, the only state where hydrogen fuel-cell cars are currently offered.

Honda, Hyundai, and Toyota have limited sales of their models—the Clarity Fuel Cell, Tucson Fuel Cell, and Mirai, respectively—to California because other states do not have sufficient fueling infrastructure.

Even California can only support fuel-cell cars in the more populous regions of the state, and early fueling stations experienced reliability issues as operators dealt with the challenge of dispensing large quantities of hydrogen to the public for the first time.

California has committed to funding a network of 100 stations through 2020.

By that time, automakers will likely still be selling fuel-cell cars in relatively small numbers, which a network of that size can handle.